General synthesis of ionic-electronic coupled two-dimensional materials

Two-dimensional (2D) AMX2 compounds are a family of mixed ionic and electronic conductors (where A is a monovalent metal ion, M is a trivalent metal, and X is a chalcogen) that offer a fascinating platform to explore intrinsic coupled ionic-electronic properties. However, the synthesis of 2D AMX2 compounds remains challenging due to their multielement characteristics and various by-products. Here, we report a separated-precursor-supply chemical vapor deposition strategy to manipulate the chemical reactions and evaporation of precursors, facilitating the successful fabrication of 20 types of 2D AMX2 flakes. Notably, a 10.4 nm-thick AgCrS2 flake shows superionic behavior at room temperature, with an ionic conductivity of 192.8 mS/cm. Room temperature ferroelectricity and reconfigurable positive/negative photovoltaic currents have been observed in CuScS2 flakes. This study not only provides an effective approach for the synthesis of multielement 2D materials with unique properties, but also lays the foundation for the exploration of 2D AMX2 compounds in electronic, optoelectronic, and neuromorphic devices.

2. The authors ufilized finite element simulafion methods to intuifively reveal the chemical reacfion environment during the CVD process, including flow fields and mass transfer processes.The reverse mass transfer of metal precursors in the small quartz tube, along with the lower concentrafion of chalcogen precursors near the metal source, ensure the stable supply of metal precursors, thereby facilitafing the preparafion of 20 types of 2D AMX2 compounds.A comprehensive discussion of the reacfion mechanism was provided.I am interested in whether the authors simulate the growth condifion at different growth temperatures?3. The synthesized 20 types of AMX2 compounds contain 9 different metal elements, including two monovalent metal ions, Cu+ and Ag+, as well as trivalent metals like Sc3+, Cr3+, Fe3+, and Ga3+, etc. Within AMX2 compounds, these metals can form bonds with chalcogen elements either in a tetrahedral coordinafion or in an octahedral coordinafion.This chemical bonding difference endows AMX2 compounds with diverse structures and intriguing properfies.Could the authors discuss the reasons behind this variafion in chemical bonding?
4. The AMX2 compounds showcased exhibit a rich variety of structures and manifest intriguing properfies, including superionic conducfivity, ferroelectricity, reconfigurable self-doping, etc. Twodimensional ferroelectric materials are of significant importance in fields such as high-density storage and reconfigurable logic devices.I am curious whether, apart from CuScS2, other AMX2 compounds also exhibit ferroelectric properfies?

Reviewer #2 (Remarks to the Author):
The authors successfully synthesized the 2D AMX2 materials through the CVD method, by using a a separated-precursor-supply route.Though similar 2D single crystals have been grown by CVT method, I am sure that it is hard to deposit by common CVD method, because of the mulfiple element components.I believe this general route itself is an important accomplishment, worthy of publicafion on its own; and the funcfional properfies such as ionic conducfivity and ferroelectricity are novel.I will suggest its publicafion, confingent upon some further clarificafion or explanafion.
-the authors have put lots of synthesis data into the supplementary materials, which I will suggest to place them into the main text, to support the novelty the work.For example, Figures S10b & 10c shows that the proposed method can deposit the homogenous single phase CuCrSe2 in a large area, which is very important when compared with tradifional method shown in Figures S3b & 3c.In addifion, the authors are suggested to show that they can have single phase in large area for other AMX2 materials.
-the ferroelectricity of CuScS2 is supported by the PFM domain switching and SHG excitafion (Figure S40).The later can tell us the Curie temperature of the ferroelectric ordering, which is the key data and is suggested to be placed into the main text.
-the authors also show the switchable photovoltaic effect of CuScS2 flakes controlled by opposite voltage poling, and they aftributed it to the ion migrafion of Cu+ towards the electrode interface.Because CuScS2 is a ferroelectric, and seems it has in-plane polarizafion as well from Figure S40.Ferroelectric polarizafion can induce intrinsic photovoltaic effect as well.One quesfion raise immediately that does this in-plane polarizafion contribute to the switchable photovoltaic effect?

Reviewer #3 (Remarks to the Author):
Mulfi-element two-dimensional materials offer an addifional degree of freedom by introducing an extra element, allowing for the modulafion of material structure and properfies, which is crucial for developing novel two-dimensional materials with superior performance.However, the synthesis of mulfi-element two-dimensional materials is currently limited by uncontrollable chemical reacfions and precursor supply.This work proposed a separated-precursors-supply chemical vapor deposifion strategy to ensure mulfiple metal precursors' stable supply and suppress the side reacfions efficiently.Then, the universal synthesis of 20 types of ternary AMX2 compounds was ulfimately achieved.Furthermore, these materials exhibit intriguing ionic-electronic properfies.This work not only offers an effecfive strategy for the synthesis of mulfi-element two-dimensional materials but also introduces a novel materials plafform.Thus, I think this work is sufficiently innovafive to be published in Nature Communicafions and suggest acceptance of this manuscript.Only some minor revisions are needed, detailed comments see below: 1.AMX2 materials exhibit rich and intriguing ionic-electronic coupled properfies, including superionic conducfivity, ferroelectricity, and reversible self-doping.These characterisfics endow the materials with significant potenfial applicafions in fields such as energy storage and informafion technology.Could the authors comment on the challenges and opportunifies to the synthesis of the AMX2 for large-scale applicafion?
2.As well known, because of the nuclear and growth processes of the products occurring on the substrate.Thus, substrate is a key factor in the growth process of target products.There are also many other kinds of substrates as SiO2/Si, Al2O3, etc. for two-dimensional materials.Herein, the authors employed mica as the collected substrate.Thus, the authors are suggested to explain why choosing mica substrates.
3.The opfical second harmonic generafion (SHG) signal is related to the non-centrosymmetric nature of ferroelectric materials.In this work, the as-synthesized CuScS2 flakes exhibit stable room-temperature ferroelectricity with a high ferroelectric Curie temperature of 370 K and has been verified by the temperature-dependent SHG measurements.Thus, the authors are suggested to provide the corresponding complete SHG spectra at different temperatures so that the readers can understand the novel ion-induced ferroelectricity befter.
4.There are some minor formafting issues in the manuscript, such as the overlap between the character "1" and the x-axis in Figure 1g, in some figure capfions, the inifial lefters are not capitalized, and missing scale bars in some AFM images in Supplementary Figure 11.

Response to Reviewer 1:
Reviewer #1: The authors present a novel chemical vapor deposition (CVD) method employing a separated-precursor-supply strategy, which effectively mitigates side reactions between metal precursors and excess chalcogen precursors, ultimately enabling the universal synthesis of 20 types of AMX2 compounds.These synthesized AMX2 compounds exhibit diverse crystal structures and demonstrate intriguing properties, including superionic conductivity, ferroelectricity, and reconfigurable selfdoping characteristics.This work has yielded significant advancements in the preparation of multi-element two-dimensional materials and the exploration of materials exhibiting ionic-electronic coupled properties, which provide a totally new platform for transistors, photodetection, and neuromorphic computing.I think this work is very timely and can broadly attract intensive attention.Therefore, I strongly recommend this work to be accepted by Nature Communications after addressing the following questions: Q1: Figure 2 illustrates the broad applicability of the synthesis strategy, wherein the synthesized 20 types of AMX2 exhibit uniformly regular morphologies.It is observed that the majority of samples shows a triangular shape, while AgInS2 presents a quadrilateral shape.Could the authors comment on the reasons underlying this morphology difference.
Answer: Thanks for the kind suggestion.Generally, the crystal face can be attributed to two aspects: thermodynamic and kinetic driving forces.When the precursors' feeding is stable and uniform during the CVD process, the morphologies of the asgrown crystals are determined by the Gibbs-Curie-Wulff law, which is the principle of surface energy minimization for predicting the equilibrium crystal shape.When a crystal face exhibits higher surface energy, it tends to grow faster, leading to a reduction in its surface area and ultimately resulting in the dominance of faces with lower surface energy (Chem.Soc.Rev. 2022, 51, 7327; Nat.Commun.2019, 10, 756).For AgInS2, we have revealed the orthorhombic phase by TEM and SAED, and due to the honeycomb-like structure of its (100) face, the crystal could show a quadrilateral shape (edge surfaces {(0-11), ( 011), (01-1), (0-1-1)}), and also can show a triangle shape (edge surfaces {(020), (01-2), (012)}).

Q2:
The authors utilized finite element simulation methods to intuitively reveal the chemical reaction environment during the CVD process, including flow fields and mass transfer processes.The reverse mass transfer of metal precursors in the small quartz tube, along with the lower concentration of chalcogen precursors near the metal source, ensure the stable supply of metal precursors, thereby facilitating the preparation of 20 types of 2D AMX2 compounds.A comprehensive discussion of the reaction mechanism was provided.I am interested in whether the authors simulate the growth condition at different growth temperatures?Answer: Thanks for the kind suggestion.Temperature is a critical factor for CVD.The nucleation and growth of crystals are closely related to the temperature.Different AMX2 have various proper growth temperatures.To provide more comprehensive support for our synthesis mechanism, we simulated the flow fields and mass transfer conditions around the small quartz tube at 600℃, 700℃, and 800℃.As Figure R1-1 shows, the flow field has no obvious change at 600℃-800℃.The reverse gas flow inside the small quartz tube always exists in this temperature range, which means that this method allows us to realize a stable supply of metal precursors when synthesizing the 20 AMX2.

Q3:
The synthesized 20 types of AMX2 compounds contain 9 different metal elements, including two monovalent metal ions, Cu + and Ag + , as well as trivalent metals like Sc 3+ , Cr 3+ , Fe 3+ , and Ga 3+ , etc. Within AMX2 compounds, these metals can form bonds with chalcogen elements either in a tetrahedral coordination or in an octahedral coordination.This chemical bonding difference endows AMX2 compounds with diverse structures and intriguing properties.Could the authors discuss the reasons behind this variation in chemical bonding?Answer: Thanks for the kind suggestion.In the AMX2 compounds, Sc 3+ , Cr 3+ , Bi 3+ , Sb 3+ exhibit octahedral coordination, Fe 3+ , Ga 3+ , In 3+ show tetrahedral coordination, and Ag + , Cu + can either occupy the octahedral or tetrahedral sites.Due to the steric effects, the oxidation state and size of the metals are important to the coordination numbers.For more transition metals tend to form up to 6 bonds.According to crystal field theory, the energy level of the e in the tetrahedral coordination is higher than the t2g band in the octahedral coordination (Figure R1-2).When the number of outmost electrons is less than 7, the filling of dxy, dxz, dyz orbitals of octahedral is more favorable.Actually, most transition metals can be induced to form octahedral coordination and examples exist for every d electron configuration ranging from group-3 (d 0 ) to group-12 (d 10 ).Based on the hybrid orbital theory, d 10 metals like Cu + , Ga 3+ , etc. have fully filled d orbital, left one ns orbital and three np orbitals to form four sp 3 hybrid orbitals, resulting in the preference of tetrahedral coordination.Some heavier group-15 elements like Sb 3+ and Bi 3+ can form d 2 sp 3 hybrid orbitals, which have octahedral coordination.In particular, the Cu + and Ag + exhibit the obvious Jahn-Teller effect, which can lower energy difference between different bonding configurations making its various occupations in AMX2 compounds.

Q4:
The AMX2 compounds showcased exhibit a rich variety of structures and manifest intriguing properties, including superionic conductivity, ferroelectricity, reconfigurable self-doping, etc. Two-dimensional ferroelectric materials are of significant importance in fields such as high-density storage and reconfigurable logic devices.I am curious whether, apart from CuScS2, other AMX2 compounds also exhibit ferroelectric properties?Answer: Thanks for the kind suggestion.In the AMX2, the origin of ferroelectricity is mainly from the non-centrosymmetric lattice occupation of Cu + and Ag + .The electric field can polarize the interlayered Cu + from tetrahedral site α to β in CuScS2, thus switching the ferroelectric polarization, thus showing ferroelectric properties.So, the AMX2 which possesses a similar layered structure characteristic of CuScS2, such as CuCrS2, CuCrSe2, AgCrS2, etc., may exhibit ferroelectricity.In addition, Wu. et al. theoretically predicted that the CuCrS2 and CuCrSe2 could show ferroelectricity when the thickness is down to nanometers (Nat.Sci.Rev. 2020, 7, 373).Additionally, we show PFM characterizations of some as-grown AMX2 in Figure R1-3.

Response to Reviewer 2:
Reviewer #2: The authors successfully synthesized the 2D AMX2 materials through the CVD method, by using a separated-precursor-supply route.Though similar 2D single crystals have been grown by CVT method, I am sure that it is hard to deposit by common CVD method, because of the multiple element components.I believe this general route itself is an important accomplishment, worthy of publication on its own; and the functional properties such as ionic conductivity and ferroelectricity are novel.I will suggest its publication, contingent upon some further clarification or explanation.

Q1:
The authors have put lots of synthesis data into the supplementary materials, which I will suggest to place them into the main text, to support the novelty the work.For example, Figures S10b & 10c shows that the proposed method can deposit the homogenous single phase CuCrSe2 in a large area, which is very important when compared with traditional method shown in Figures S3b & 3c.In addition, the authors are suggested to show that they can have single phase in large area for other AMX2 materials.
Answer: We sincerely appreciate the valuable comments.We have reorganized our Figure 1.Some important synthesis data have been added in the latest version (Figure R2-1).And, to better show the uniform synthesis of AMX2, we add the larger area optical images of all 20 AMX2 in the Supporting information (Figure R2-2).

Q2:
The ferroelectricity of CuScS2 is supported by the PFM domain switching and SHG excitation (Figure S40).The later can tell us the Curie temperature of the ferroelectric ordering, which is the key data and is suggested to be placed into the main text.
Answer: Thanks for the kind suggestion.The ferroelectric Curie temperature reflects the stability of the ferroelectric ordering.To help readers get a better understanding, we have added the temperature-dependent SHG data in the revised manuscript(Figure R2-3).

Q3:
The authors also show the switchable photovoltaic effect of CuScS2 flakes controlled by opposite voltage poling, and they attributed it to the ion migration of Cu + towards the electrode interface.Because CuScS2 is a ferroelectric, and seems it has in-plane polarization as well from Figure S40.Ferroelectric polarization can induce intrinsic photovoltaic effect as well.One question raise immediately that does this in-plane polarization contribute to the switchable photovoltaic effect?Answer: Thanks for the kind suggestion.Based on the atomic structure of the CuScS2, the Cu + will take place a small lateral displacement along with the vertical displacement when switching the ferroelectric polarization of the CuScS2, which may induce the in-plane (IP) polarization.So, we have tried our best to investigate the IP polarization of the as-grown CuScS2 nanosheets.However, maybe the IP polarization is too weak to be detected by the PFM, as Figure R2-4 shows, there is no typical IP PFM phase and amplitude hysteresis loop.When discussing the mechanism of the switchable photovoltaic behavior of CuScS2 devices, it is important to consider that there is no obvious short current when the device is at its initial state, which means the bulk photovoltaic effect is negligible, representing the small contributions of the IP polarization of the CuScS2.On the other hand, due to the low migration barrier (0.24 eV/atom), the Cu + ions prefer the long-distance migration under in-plane electric polarization, which has been proved by the cross-sectional EDS mapping of CuScS2 device after polarization (Figure S45).

Response to Reviewer 3:
Reviewer #3: Multi-element two-dimensional materials offer an additional degree of freedom by introducing an extra element, allowing for the modulation of material structure and properties, which is crucial for developing novel two-dimensional materials with superior performance.However, the synthesis of multi-element twodimensional materials is currently limited by uncontrollable chemical reactions and precursor supply.This work proposed a separated-precursors-supply chemical vapor deposition strategy to ensure multiple metal precursors' stable supply and suppress the side reactions efficiently.Then, the universal synthesis of 20 types of ternary AMX2 compounds was ultimately achieved.Furthermore, these materials exhibit intriguing ionic-electronic properties.This work not only offers an effective strategy for the synthesis of multi-element two-dimensional materials but also introduces a novel materials platform.Thus, I think this work is sufficiently innovative to be published in Nature Communications and suggest acceptance of this manuscript.Only some minor revisions are needed, detailed comments see below: Q1: AMX2 materials exhibit rich and intriguing ionic-electronic coupled properties, including superionic conductivity, ferroelectricity, and reversible self-doping.These characteristics endow the materials with significant potential applications in fields such as energy storage and information technology.Could the authors comment on the challenges and opportunities to the synthesis of the AMX2 for large-scale application?Answer: Thanks for your kind suggestion.Developing large-scale array devices is crucial for practical applications in the future.The development of CVD synthesis methods for large-area 2D AMX2 plays a key role.Due to the crystal structures of some AMX2 are composed of [CrS2], [ScS2], [ScSe2], and [BiSe2], etc., we can draw inspiration from the recent advances in the preparation of wafer-scale transition metal dichalcogenides, for example the substrate-engineering strategies (Chem.Soc.Rev. 2023, 52, 1650).However, ensuring precise control over stoichiometry in large-scale synthesis still be challenging due to the issues such as non-uniform mixing and variations in reaction conditions.Combining the separated-precursor-supply idea of this work and substrate-engineering strategies, we may realize the wafer-scale synthesis of the AMX2 materials in the future.On the other hand, we believe these novel 2D AMX2 will attract intensive attention in photoelectronic artificial neural networks and integrated sensing-computing devices based on their controllable ion migration properties.

Q2:
As well known, because of the nuclear and growth processes of the products occurring on the substrate.Thus, substrate is a key factor in the growth process of target products.There are also many other kinds of substrates as SiO2/Si, Al2O3, etc. for two-dimensional materials.Herein, the authors employed mica as the collected substrate.Thus, the authors are suggested to explain why choosing mica substrates.
Answer: Thanks for the kind suggestion.Although SiO2/Si and Al2O3 have been used as the collected substrate for several two-dimensional structures, they are not the ideal substrates for all the two-dimensional structures, especially for the non-van der Waals structure materials.The reason for choosing mica substrates is that layered mica substrate is considered to be a perfect van der Waals epitaxy substrate for growing 2D materials due to the atomically smooth surface.It not only avoids the strict requirement of lattice matching between the epitaxial two-dimensional material and substrates but also facilitates the fast migration of the adatoms during the growth (Adv.Funct.Mater.2018, 28, 1800181; Adv.Mater.2017, 29, 1703122).

Q3:
The optical second harmonic generation (SHG) signal is related to the noncentrosymmetric nature of ferroelectric materials.In this work, the as-synthesized CuScS2 flakes exhibit stable room-temperature ferroelectricity with a high ferroelectric Curie temperature of 370 K and has been verified by the temperaturedependent SHG measurements.Thus, the authors are suggested to provide the corresponding complete SHG spectra at different temperatures so that the readers can understand the novel ion-induced ferroelectricity better.
Answer: Thanks for the kind suggestion.We have provided every single SHG spectrum at different temperatures in the revised supporting information.Q4: There are some minor formatting issues in the manuscript, such as the overlap between the character "1" and the x-axis in Figure 1g, in some figure captions, the initial letters are not capitalized, and missing scale bars in some AFM images in Supplementary Figure 11.

Answer:
We are sorry for the careless mistakes.We have checked the whole manuscript again and corrected the problems.The changes have been highlighted in red.

Figure R1- 1 .
Figure R1-1.The velocity distribution around the small quartz tube at different temperatures.a-c, Distribution of Velocity u in the small quartz tube at 600℃, 700℃, and 800℃ respectively.b, The variation curve of Velocity u along the dashed line in (a)-(c).

Figure R1- 2 .
Figure R1-2.Schematic images of octahedral and tetrahedral coordination and energy levels of d-orbital electrons based on the crystal field theory.

Figure R2- 3 .
Figure R2-3.The ionic and electronic properties of two representative assynthesized 2D AMX 2 compounds.a, d, g, Schematic diagrams of the superionic conductor (a), ferroelectricity (d), and reversible self-doping (g) properties that arise from the ionic-electronic coupling effects.b, The impedance spectroscopic measurement for AgCrS2 nanosheets with different thickness.c, The comparison of ionic conductivity with other reported superionic conductors.e, The PFM phase and amplitude hysteresis loop of CuScS2 nanosheet, the inset is the ferroelectric domains of CuScS2 nanosheet after forward and reverse DC bias polarization.f, The temperature-dependent SHG measurement of a CuScS2 nanosheet.h, i, Memristor behavior (h) and switchable photovoltaic behavior under illumination (λ=532 nm, 256.6 mW/cm 2 ) (i) after positive/negative pulse polarization of the CuScS2-based device.

Figure R3- 1 .
Figure R3-1.Temperature-dependent SHG measurement of an as-grown CuScS 2 nanosheet.a, The spectra of SHG signal at different temperatures.b, Schematic diagram of the arrangement of Cu + ion in CuScS2 structure when the temperature is below and beyond the TC.